Apparatus for chemical vapor deposition of aluminum oxide

ABSTRACT

An apparatus in a chemical vapor deposition (CVD) system monitors the actual wafer/substrate temperature during the deposition process. The apparatus makes possible the production of high quality aluminum oxide films with real-time wafer/substrate control. An infrared (IR) temperature monitoring device is used to control the actual wafer temperature to the process temperature setpoint. This eliminates all atmospheric temperature probing. The need for test runs and monitor waters as well as the resources required to perform the operations is eliminated and operating cost are reduced. High quality, uniform films of aluminum oxide can be deposited on a silicon substrates with no need for additional photolithographic steps to simulate conformality that are present in a sputtered (PVD) type application. The result is a reduction in required process steps with subsequent anticipated savings in equipment, cycle time, chemicals, reduce handling, and increased yield of devices on the substrate. The apparatus incorporates a heated source material, heated delivery lines, heated inert gas purge lines, a pressure differential mass flow controller, a control system with related valving, and a vacuum process chamber with walls that are temperature controlled as a complete source delivery system to accurately and repeatably provide source vapor for LPCVD deposition of aluminum oxide onto silicon substrates.

FIELD OF THE INVENTION

The present invention generally relates to the chemical vapor deposition (CVD) of dense thin aluminum oxide (Al₂ O₃) films on silicon substrates in the manufacture of semiconductor devices and, more particularly, to an improved method and apparatus for the low pressure chemical vapor deposition (LPCVD) of aluminum oxide films using aluminum tri-isopropoxide (Al(OC₃ H₇)), known in the art as ATI, as a precursor to aluminum oxide in a high volume manufacturing environment.

BACKGROUND OF THE INVENTION

The utility of CVD Al₂ O₃ as a reactive ion etch (RIE) stop layer (during tungsten contact stud patterning) has been clearly demonstrated. For this application, a dense aluminum oxide film is defined as having the following characteristics:

.Index of refraction: 1.59-1.62

.Etch rate (3.6% phosphoric acid at 75° C.):<25 nm/min

.Hydroxyl content (OH, by infrared (IR) spectroscopy, absorbance per micron of film thickness at 3500 cm-⁻¹):<0.005

.Shrinkage of film thickness upon annealing (30 min. at 500° C.):<1.5%

The literature indicates that aluminum tri-isopropoxide (Al(OC₃ H₇)), known in the art as ATI, is a viable sub-500° C. Al₂ O₃ precursor. See, for example, J. A. Aboaf, J. Electrochem. Soc., 114, 948 (1967), J. Fournier et al., Mat. Res. Bull., 23, 31 (1988), J. Kwon, J. Saraie and Y. Yodogawa, J. Electrochem Soc., 132, 890 (1985), H. Mutoh et al., J. Electrochern. Soc., 122, 987 (1975), and R. W. J. Morssinkhof et al., "Mechanistic Aspects of the Deposition of Thin Alumina Films Deposited by MOCVD", paper presented at Spring Meeting of the Materials Research Society, San Francisco, Calif. (1990). In these references, ATI is utilized under conditions in which 1) only bubbling is used to deliver ATI to the reactor, 2) a variety of ATI operating temperatures (78°-170° C.) is practiced, 3) deposition repeatability is not demonstrated, and 4) only a single substrate requires coating. However, while the literature describes the use of ATI for Al₂ O₃ deposition, the literature describes bubbling techniques using an ATI carrier but fails to present a workable manufacturing process. In fact, the bubbling technique described in the literature will not work in a manufacturing environment.

During the aluminum oxide low pressure chemical vapor deposition (LPCVD) process, substrate temperature is critical to the deposition of a high quality aluminum oxide film. Low wafer temperature typically yields a film with high etch rates and low density. If the substrate temperature is not controlled from run to run, the etch rate can vary and subsequent etch processes cannot be accurately controlled in a batch process. Using an embedded thermal measurement device such as a resistance thermal device (RTD) or thermocouple within the susceptor or chuck provides no direct measurement of the wafer/substrate temperature, and there is no guarantee that consecutively processed substrates/wafers will achieve the same temperature and thus the films will have different etch rates. Although the substrate/wafer rests on and may be mechanically gripped by the heated susceptor/chuck, the thermal transfer at process pressure between the substrate/wafer and the susceptor/chuck is not predictable and may lead to large differences between the susceptor/chuck temperature and the successive processed wafers/substrates.

There is no current means to insure wafer/substrate temperature during the LPCVD process. Instead, various external means of calibrating the temperature controller are employed. One such technique requires the process chamber to be vented to atmosphere and a thermocouple probe placed against the wafer susceptor/chuck face to measure the face temperature. Adjustments are then made to the susceptor/chuck temperature controller based on the readings from the thermocouple probe. These types of procedures do not take into account chuck face emissivity, radiation heating effects with the system at process pressure, and/or the wafer/substrate emissivity when the wafer/substrate is place on the chuck/susceptor. The obvious result is a series of "test" runs that are used to measure the etch rate on monitor wafers prior to processing of actual product wafers. This becomes extremely costly with respect to the resource time required, the expense of monitor wafers/substrates, the loss of process tool availability of not only the LPCVD system, but also wet etch and inspection/measurement equipment.

A plasma vapor deposition (sputtered) technique may be employed with several photolithography and wet etch steps to simulate the conformal coverage. However, the many steps increases equipment cost, cycle time, chemical, handling, and additional resources.

An improperly designed delivery system for transporting a low pressure reactive vapor from a heated source vessel to a vacuum processing chamber will lead to vapor decomposition or condensation of the vapor. Either of which result in the reactive vapor byproducts depositing within the delivery system causing restrictions and eventual clogging of the delivery system.

Transporting a low pressure reactive vapor from a heated source vessel to a vacuum processing chamber can lead to decomposition or condensation of the vapor material onto the walls of the supply tubing or flow controlling devices. To initiate and maintain a constant flow of a low pressure vapor material, a heated source vessel is used to provide adequate vapor pressure. An unheated delivery system will allow the vapor to condense within the delivery system causing restrictions and eventual clogging of the delivery system. Conversely, if heating in the delivery system is too high, decomposition of the vapor will occur resulting in poor deposited film quality, inferior etch stop quality within the wafer/substrate, and eventual plating out to form blockages within the delivery system.

Stagnant source chemical trapped in the delivery system during idle periods when there is no flow may also decompose and plate out into the delivery system eventually causing flow restriction and ultimately system blockage. Vacuum pumping of the delivery lines themselves has been demonstrated to be ineffective in removing the reactive source vapor and resultant system blockage. An additional problem can also be encountered due to a surface film being formed in the source vessel that limits the vapor generated and starves the process for reactive chemical. Without correctly designed substrate temperature measurement and control, inert gas purging, and agitation of the source chemical, delivery of the vapor to the process chamber is not a reliable or manufacturable process.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a means to measure the actual wafer/substrate temperature during the chemical vapor deposition of aluminum oxide.

It is another object of this invention to provide a thermally balanced, inert gas purged, and temperature controlled vapor delivery system eliminating delivery system blockages caused by condensation and/or decomposition of reactant vapor within the vapor delivery system.

It is another object of this invention to eliminate poor film quality deposition on the first wafer of a production ran.

It is another purpose of this invention to provide a reproducible and manufacturable deposition process that provides a high quality and conformal aluminum oxide film using low pressure chemical vapor deposition (LPCVD) on a silicon substrate or equivalent.

According to the invention, high quality, uniform films of aluminum oxide can be routinely deposited on a silicon substrate with no need for additional photolithographic steps to simulate conformality that are present in a sputtered (PVD) type application. The result is a reduction in required process steps with subsequent anticipated savings in capital equipment, cycle time, chemicals, and human resources.

The invention incorporates into a typical LPCVD reactor, a central temperature controller, an accurate heating system for the source material, an accurately heated source delivery system, an accurately heated inert gas purge system, accurately heated gas dilution gas lines, an accurately heated vacuum foreline, an accurately heated throttle valve, an accurately heated pressure differential based mass flow controller and accurate temperature control of the vacuum chamber wall temperature. The invention also features real time substrate temperature control utilizing IR temperature monitoring and feedback control and a Fourier transform infrared spectrometer (FTIR) vapor analysis system to control reactant vapor flow.

The apparatus used in the practice of the invention incorporates a heated source material, heated delivery lines, heated inert gas purge lines, a pressure differential mass flow controller, a control system with related valving, and a vacuum process chamber having walls that are temperature controlled as a complete source delivery system to accurately and repeatably provide source vapor for LPCVD deposition of aluminum oxide onto silicon substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

FIG. 1 is a block diagram showing the major components of the apparatus for depositing thin films of aluminum oxide on wafers according to a preferred embodiment of the invention;

FIG. 2 is a flow diagram showing the control of the apparatus shown in FIG. 1 by the microprocessor controller;

FIG. 3 is a functional block diagram of the infrared (IR) wafer temperature control system used in the low pressure chemical vapor deposition (LPCVD) system shown in FIG. 1; and

FIG. 4 is a block diagram of the LPCVD system shown in FIG. 1 illustrating in more detail the various components of the system for real time temperature control.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

Referring now to the drawings, and more particularly to FIG. 1, there is shown an apparatus for depositing aluminum oxide on a substrate according to a preferred embodiment of the invention. A pressure vessel 11 contains aluminum tri-isopropoxide (ATI) source material from which a source chemical vapor is produced. A delivery manifold 12, including a pair of valves 12₁, and 12₂, is connected to the pressure vessel 11 and has first and second outputs respectively connected to these two valves. The valves 12₁ and 12₂ of the delivery manifold selectively deliver the ATI source chemical vapor to the first or second output. An isothermal oven 13 contains the pressure vessel 11 and the delivery manifold 12 for heating them and maintaining them at an accurate, controlled temperature. Sensors 14 and 15 respectively monitor pressure and temperature within the pressure vessel 11 and communicate this data to the process controller via signal cables 14₁ and 15₁, respectively.

A chemical vapor deposition (CVD) process chamber 16 has wall temperature control by the process controller 17, as indicated by control line 18. The CVD process chamber 16 also has at least one gas injector nozzle 19 for injection of gas into the chamber and a heated susceptor or chuck 20 which holds a workpiece 10, such as a silicon wafer substrate, on which aluminum oxide films are to be deposited. Insulated and temperature controlled exhaust lines 21₁ and 21₂ are respectively connected to the first output of said delivery manifold 12 and to CVD process chamber 16 for providing a temperature controlled exhaust from the delivery manifold and the CVD process chamber to a vacuum pump 22. A temperature controlled delivery line 23 is connected at the first end thereof to the second output of said delivery manifold 12. The delivery line 23 has a cross section so as not to restrict the mass flow of the ATI source chemical vapor for a desired deposition rate of aluminum oxide on a wafer. A temperature controlled pressure differential mass flow controller (PDMFC) 24 is ultimately connected to the second end of the delivery line 23 and, under the control of process controller 17, provides accurate flow of the ATI source chemical vapor through the delivery line 23.

A premix chamber 25 including a heated and insulated valve premix manifold assembly 26 is connected to the pressure differential mass flow controller 24. The premix manifold assembly 26 has first and second outputs. The first output of said premix manifold assembly 26 comprises a chamber for allowing the ATI source chemical vapor to be mixed with a mass flow controlled, preheated gas, such as argon from source 27. This gas is delivered via mass flow controller (MFC) 28 to the premix manifold assembly 26. The first output of the premix manifold assembly 26 is connected by line 29 to the gas injector nozzle 19 of the CVD process chamber 16 for injection of the mixed ATI source chemical vapor and diluent gas therein. The second output of the premix manifold assembly 26 is connected to the exhaust line 21₃.

The process controller 17 is a microprocessor based controller and is connected to the delivery manifold 12, the oven 13, the pressure vessel monitoring sensors 14 and 15, the exhaust lines 21₁, 21₂, and 21₃, heated throttle valve, the delivery line 23, the pressure differential mass flow controller 24, the mass flow controller 28, the walls of CVD reactor 16, and the premix manifold assembly 26 for controlling the pressure and the temperature of the ATI source chemical vapor being delivered throughout the system to be substantially constant thereby minimizing potential condensation and/or decomposition of the ATI source chemical vapor during delivery. The process controller 17 further controls the delivery of the ATI source chemical vapor from the pressure vessel 11 to the exhaust lines 21₁, and 21₃, the delivery line 23, the premix manifold assembly 26, and the CVD process chamber 16 according to prescribed deposition process parameters.

The delivery line 23 leads to an infrared (IR) cell 30. A Fourier transform infrared (FTIR) spectrometer 31, or other IR sensing device proximate to the IR cell 30, analyzes the full concentration of vapor flowing through the IR cell 30 before it travels to the chemical vapor deposition chamber 16. The FTIR spectrometer 31 sends information to the process controller 17 which controls the valve of the premix manifold assembly 26. The process controller 17 operates the valve of the premix manifold assembly 26 based upon the information it receives from the FTIR spectrometer 31. The delivery line 23 leads from the IR cell 30 through the pressure differential mass flow controller (PDMFC) 24 to the valve of the premix manifold assembly 26. The outputs from this valve are represented by line 29 leading to the CVD process chamber 16 and the exhaust line 21₃ leading to the system pump 22.

When the reactant vapor flowing in the IR cell 30 is analyzed by the FTIR spectrometer 31, information is given to the process controller 17 indicating whether contaminants or decomposition products are present. A composition of reactant vapor which is substandard for quality Al₂ O₃ film deposition will cause the process controller 17 to operate the valve of the premix manifold assembly 26 to switch such that the ATI chemical source vapor will proceed through exhaust line 21₃ to pump 22, rather than through line 29 to the chemical vapor deposition chamber 16. If the composition of the ATI chemical source vapor is appropriate, the valve of the premix manifold assembly 26 will be operated to direct the ATI chemical source vapor through line 29 to nozzle 19 in the CVD process chamber 16 and proceed to react on workpiece 10.

The process controller 17 operates to control the temperature of the ATI in pressure vessel 11 during periods of no production demand at a temperature no less than 90° C. but not greater than 95° C. During periods of no production demand, the ATI is stored under vacuum. The process controller 17 raises the ATI temperature to a temperature no greater than 125 ° C. in preparation for a period of production demand and maintains the ATI during periods of production demand at a temperature not greater than 125° C. The ATI chemical vapor is transported to the CVD process chamber 16 via direct evaporation. To improve delivery of the ATI chemical vapor during periods of production demand, the ATI pressure vessel 11 may be agitated. The ATI chemical vapor is directed to the reactor bypass exhaust line 213 in lieu of nozzle 19 in CVD process chamber 16 during a start up period, and the flow to the bypass exhaust line 21₃ is maintained until the concentration of ATI chemical vapor in the delivery line 23 to the reactor is greater than a prescribed level, typically greater than 99%, as determined by the FTIR system. The ATI chemical vapor is directed to nozzle 19 in the CVD process chamber 16 through line 29 after the concentration of ATI chemical vapor in the reactor supply line has reached a prescribed level, typically greater than 99%, for deposition of aluminum oxide on the workpiece 10. The temperature of the ATI chemical vapor is again reduced to about 90° C. after termination of the production demand period.

FIG. 2 is a flow diagram showing the process controlled by the microprocessor based process controller 17. On start up, the process controller 17 determines whether there are wafers ready for processing, as indicated by decision block 41. If not, the system idles at function block 42 until wafers are present for processing. Once wafers are ready for processing in CVD process chamber 16, the oven 13 temperature is raised to 125° C. and stabilized, as indicated by function block 43. Then, the ATI is flowed through delivery line 23 to the IR cell 30 and, from there, via the valve in premix manifold assembly 26 to the exhaust line 213, bypassing the process chamber 16 as indicated in function block 44. Based on the output of FTIR spectrometer 31, the process controller 17 determines in decision block 45 whether the concentration of the reactant gas is sufficiently pure. If it is, the valve in the premix manifold assembly 26 is operated to flow the ATI vapor to the CVD process chamber 16 in function block 46. The flow of ATI to the CVD process chamber 16 continues until processing of the wafers is completed, as determined in decision block 47. At that point, the valve in premix manifold assembly 26 is actuated, the oven 13 temperature is reduced to 90° C., as indicated in function block 48, and pump out of ATI via exhaust line 21₁ is accomplished during idle.

Referring now to FIG. 3, wherein like reference numerals as used in FIG. 1 represent the same or similar components, there is shown the automatic IR wafer temperature control system according to one aspect of this invention. The IR temperature control system includes a wafer present sensor 58 which detects the presence of a wafer/substrate 10 on a wafer chuck 20 within an LPCVD reaction chamber. The sensor 58 communicates electrically with a wafer present sensor circuit 51, typically an amplifier and threshold circuit providing a predetermined output indicative of the presence of a wafer/substrate 10 within the reaction chamber. The output of the wafer present sensor circuit 51 is connected to a selector circuit 52 which is connected to process controller 17. The process controller 17 controls the heating of the wafer chuck 20 by means of the resistance heater 60. The selector circuit 52 has a connection to receive direct information from the IR temperature monitor circuit 55, which in turn is connected to an IR sensor 56. A third input line to the selector circuit 52 provides a connection to the resistance thermal device (RTD) signal conditioning circuit 54, which in turn is connected to resistance thermal device sensor 61 imbedded in the wafer chuck 20. The RTD circuit and device sensor are conventional in the prior art.

In operation, the temperature control portion of this invention works as follows. During idle time with no wafer/substrate 10 on the wafer chuck 20, the wafer present sensor 51 sends a signal to the selector 52 telling the process controller 17 to allow the RTD circuit 54 to control the wafer chuck temperature. When a wafer/substrate 10 is placed onto the wafer chuck 20, the wafer present sensor 51 then notifies the selector 52 to switch temperature control to the IR temperature monitor circuit 55. The IR temperature monitor circuit 55 is then used by the process controller 17 to adjust the temperature of the wafer chuck 20 to maintain the process temperature set point based on the actual wafer temperature. When the LPCVD process has been completed, the wafer/substrate 10 is removed from the wafer chuck 10 and control of the wafer chuck temperature is returned to the RTD circuit 54.

With reference again to FIG. 1, to establish pure chemical flow, the heated source 11 chemical (aluminum tri-isopropoxide or ATI) is delivered through the manifold 12 to the heated exhaust line 21₃ via delivery line 23. The FTIR spectrometer 31 monitors the purity of the ATI vapor. Once the desired purity level has been established, the FTIR spectrometer 31 informs the process controller 17 that the ATI vapors can be redirected from the heated exhaust line 21₃ to the process chamber 16.

The process controller 17 then polls the automated IR temperature monitor circuit 55 (see FIG. 3), and if the IR temperature monitor circuit 55 indicates the wafer/substrate temperature is within tolerance, the process controller 17 then switches the ATI flow from the heated exhaust line 213 to the process chamber 16. If the wafer/substrate 10 is not at the prescribed temperature range, or if some portion of the temperature controlled delivery or exhaust system is not at the prescribed temperature the process controller 17 continues to poll the IR temperature monitor circuit 55, and the temperature controlled delivery and exhaust system until an acceptable signal is received or an alarm is generated.

In FIG. 4, where again like reference numerals designate like or similar components as shown in FIG. 1, the ATI source material is heated in oven 13 to obtain the required vapor pressure. Temperature monitoring sensor 15 feeds data to the process controller 17 (FIG. 1) which, in turn, maintains a correct source chemical temperature. The reactive vapor is then either contained in the pressure vessel 11 until it reaches the appropriate temperature, or the reactive vapor is allowed to flow to the vacuum pump 22 (shown in FIG. 1) through the temperature controlled exhaust line 21₃ or to the CVD process chamber 16 through a separate temperature controlled line 29. At the end of the deposition step, the source is again isolated and inert gas passed through the vapor wetted lines, thus removing stagnant reactive vapor.

More particularly, the inert gas from source 27 (FIG. 1) is supplied to each of valves 71, 72 and 79. Valve 71 is connected via a mass flow controller 28₃ and a pressure sensor 73 to a valve 74. Valve 72 is connected to a mass flow controller 28₂, and valve 79 is connected to mass flow controller 28₁. The mass flow controllers 28₁, 28₂ and 28₃ are represented schematically as the mass flow controller 28 shown in FIG. 1. The valve 74 and the mass flow controller 28₂ are connected to the valves of the delivery manifold 12 within the oven 13. Specifically, valve 74 is connected to valves 12₃ and 12₄, and mass flow controller 28₂ is connected to valve 12₆. Valve 12₃ is connected at an input of pressure vessel 11. Another valve 12₅ is connected at an output of pressure vessel 11, and this valve 12₅ is connected at a common juncture with valves 12₄ and 12₁. The pressure sensor 14 is connected to this common juncture and to valve 12₂ which is also connected to valve 12₆. The common juncture between valves 12₂ and 12₆ is connected to the delivery line 23₁. In addition to these valves, a valve 78 is connected between the mass flow controller 28₁ and the premix chamber 25. The valves 75 and 77 are part of the premix manifold assembly 26 (FIG. 1 ). Valve 76 is connected in exhaust line 21₄.

In operation during nondeposition periods, the diluent gas, preferably argon, flows through all lengths of the supply and exhaust lines except for the pressure vessel 11 and is controlled by the valve system and mass flow controllers 28₂ and 28₃. Flow of diluent gas through the pressure vessel 11 is prevented by closing valves 12₃ and 12₅. With the use of a heated diluent gas flowing through the temperature controlled supply lines, all source chemical is purged from the lines and decomposition is prevented.

Upon initiation of source heat up prior to a deposition cycle the process controller 17 performs the following purge sequence to insure there is no film or scum across the ATI in source vessel 11. Valves 71, 74, 12₃, 12₅, 12₁, and 21₄ are opened. The pressure sensor 73 is connected to the process controller 17, and the mass flow controller 28₃ is controlled by the process controller 17. Inert gas (such as argon) is allowed to flow thru this valve and vent arrangement to the vacuum pump 22 for a preset time to break apart any film or scum that has formed on the ATI melt within the source vessel 11.

When source delivery is required, the process controller 17 closes the appropriate valves, 12₃, 12₄, 12₁, and 12₆, and the source flow is initiated to the exhaust through the temperature controlled delivery lines to mass flow controller 24 and, via valves 75 and 76, to the exhaust lines 21₂, 21₃, and 21₄ while also opening valve 78 to establish diluent flow through mass flow controller 28₁ to the premix chamber 25. Once stable flow is established through mass flow controller 24, the FTIR spectrometer 31 (FIG. 1 ) indicates pure chemical flow and said substrate temperature sensor 55 (FIG. 3) indicates a substrate temperature within a prescribed process tolerance, valves 75 and 76 are closed and valve 77 is opened introducing the ATI vapor into the premix chamber 25, so as to mix the ATI vapor with the diluent gas. Once deposition is completed, the source supply is shut off at valve 12₅. Valves 71, 72, 74, 124, 12₁ and 12₆ are then opened. PDMFC 24 is also given a full open command from the process controller 17. This returns the diluent gas flow through the source delivery lines to the vent system and vacuum pumps, purging the delivery and vent lines of residual gases. Any residual source chemical carried through the delivery and vent lines is deposited in the cold trap at the end of the heated exhaust line 21₂. This protects the down stream vacuum system from physical damage from the decomposed source chemical.

The control of the delivery line temperature is critical to long term repeatable operation of the system. All wetted surfaces of the delivery system must be at the same temperature as the prior section or at some slightly elevated temperature. A reduction in temperature from the prior section will initiate condensation of the source material. Conversely, if an incremental increase in the temperature gradient from section to section is too great, source decomposition or "plating out" is realized within the delivery line, again causing restriction and eventual clogs. In the preferred practice of the invention, a temperature gradient of +1° C. for each section is adequate to eliminate condensation and prevent decomposition plate out over a reasonable length of delivery line. The invention uses a zoned heating approach where individual sections of delivery lines are carefully heat taped, insulated, and independently temperature controlled and monitored.

Referring again to FIG. 4, each zone is outlined and has its own temperature controller and set point. With this technique, source delivery can be reliably obtained. For example, if the temperature of the heated oven 13 is selected to be 120° C., then the temperature of the heated source delivery system 23₁ leading to the mass flow controller 24 should be 120°+1° C. or 121° C. Likewise, the temperature of the exhaust line 21₁ should be 121° C. The section of heated source delivery line 23₂ which leads to the injector nozzle should be 122° C. The CVD process chamber and exhaust line 21₃, like the heated source delivery line 23₂, should also be 122° C. Finally, the temperature of the exhaust line 21₂ to the vacuum pump should be 123° C.

While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims. 

Having thus described our invention, what we claim as new and desire to secure by Letters Patent is as follows:
 1. A method of depositing a dense aluminum oxide upon a substrate in a low pressure chemical vapor deposition system including a vacuum processing chamber having at least one gas injector nozzle for injection of gas into the chamber, a heated susceptor upon which is placed said substrate, and temperature controllable walls, said method comprising the steps of:a) providing a source chemical vapor delivery system having a premix chamber connected to the processing chamber, the source chemical vapor delivery system generating and delivering an ATI chemical source vapor of aluminum tri-isopropoxide (ATI) from an ATI source material; b) heating the ATI source material to a temperature sufficient to provide a flow of said ATI chemical source vapor; c) controlling the flow of ATI chemical source vapor in the source chemical vapor delivery system; d) maintaining a process pressure within the processing chamber and the source chemical vapor delivery system at a prescribed pressure; e) monitoring a purity of the ATI chemical source vapor as the vapor is delivered and providing an indication of either a first chemical flow or a second chemical flow, wherein said first chemical flow has a higher purity than that of said second chemical flow; f) monitoring the substrate temperature with a temperature sensor to provide an indication of the substrate temperature; g) selectively delivering ATI chemical source vapor to a heated exhaust line of the source chemical vapor delivery system when the purity monitoring step indicates said second chemical flow or selectively delivering ATI chemical source vapor to the premix chamber of the source chemical vapor delivery system when said purity monitoring step indicates said first chemical flow and said temperature sensor indicates a substrate temperature within a prescribed process tolerance; h) mixing a preheated inert gas with the flow of ATI chemical source vapor in the premix chamber of the source chemical vapor delivery system to produce a gas mixture, the flow of inert gas being mass flow controlled; and i) injecting the gas mixture into the process chamber so that the ATI chemical source vapor reacts on the substrate to form an aluminum oxide film thereon.
 2. The method recited in claim 1 wherein the step f) of monitoring the substrate temperature is performed using an infrared sensor.
 3. The method recited in claim 1 further comprising the step of controlling the pressure and the temperature of source chemical vapor being delivered throughout the system to be substantially constant thereby minimizing potential condensation and/or decomposition of the source chemical vapor during delivery.
 4. The method recited in claim 3 wherein the temperature of the source chemical vapor is not greater than 125° C.
 5. The method recited in claim 4 wherein the step b) of heating the ATI source material is performed in an oven and the system is divided into zones between the oven and the vacuum process chamber further comprising the step of controlling the temperature of each succeeding zone from the oven to the vacuum process chamber to +1° C. greater than a preceding zone. 